PHP also supports regular expressions using a Perl-compatible syntax
using the PCRE functions. Those functions
support non-greedy matching, assertions, conditional subpatterns, and a
number of other features not supported by the POSIX-extended regular
expression syntax.

Warning

These regular expression functions are not binary-safe. The PCRE functions are.

Regular expressions are used for complex string manipulation.
PHP uses the POSIX extended regular expressions as defined by POSIX
1003.2. For a full description of POSIX regular expressions see the » regex
man pages included in the regex directory in the PHP distribution. It's
in manpage format, so you'll want to do something along the lines of
man /usr/local/src/regex/regex.7 in order to read it.